1. Field of the Invention
The present invention relates a two-plate type of light receiving unit and an image taking apparatus having the light receiving unit.
2. Description of the Related Art
In the event that upon receipt of light of the subject a color image according to the subject light is displayed, the color image is displayed on a display screen in such a way that the subject light is separated into R-color, G-color and B-color, that is, three primary colors of light, so that image signals associated with the respective colors are created, and then elements associated with R-color, G-color and B-color, which are arranged on the display screen on a two-dimensional basis, are driven by the image signals.
To separate the subject light into R-color, G-color and B-color of three primary colors, according to a single-plate type of light receiving unit, R, G and B filters for color spectrum are arranged on a light receiving plane of a single imaging device, and according to a three-plate type of light receiving unit, a color separation mirror for spectrum (generally, as the color separation mirror, a dichroic mirror is often used and thus the color separation mirror is referred to as the dichroic mirror hereinafter) is disposed before each of three imaging devices (cf. for example, Japanese Patent Application Laid Open Gazette TokuKai Hei. 6-281881, and Japanese Patent Application Laid Open Gazette TokuKai 2000-201290).
According to the single-plate type of light receiving unit, the R, G and B filters are used to perform the color separation, and thus it involves a defect that a transmission loss of the filters causes sensitivity of the imaging device to lower, but it is relatively simple in the structure. Therefore, the single-plate type of light receiving unit is often used in a digital camera. O the other hand, according to the plural-plate type of light receiving unit, it involves a defect that the number of parts is increased to bring about complication of the structure, and as a result this may bring about increment of the cost. However, a dichroic mirror is used without the use of the color filters to form an image of the subject on the imaging device, so that a high sensitivity of image signal can be obtained. Thus, the plural-plate type of light receiving unit is used in a camera for broadcast.
Further, there is proposed a technology of improving the sensitivity of light receiving of the imaging device in such a way that antireflection films for preventing reflections of R-light, G-light and B-light are formed on light receiving surfaces of three imaging devices so that spectrums (R-light, G-light and B-light), which reach the light receiving surfaces of three imaging devices passing through the dichroic mirror, respectively, are received on the light receiving surfaces of the imaging devices with high sensitivity (cf. for example, Japanese Patent Application Laid Open Gazette TokuKai 2003-309858).
By the way, according to an image taking apparatus having a single-plate type of light receiving unit, it would happen that when a high-speed shutter is used to perform photography, a deterioration of the sensitivity of the imaging device causes a lack of exposure time, and as a result, this brings about a lack of brightness. In view of the foregoing, it is considered that a three-plate type of light receiving unit, which is higher in sensitivity than the single-plate type of light receiving unit, is used instead of the single-plate type of light receiving unit, in order that a sufficient light quantity is supplied to the light receiving surface of the imaging device even if the high-speed shutter is used. However, the use of the three-plate type of light receiving unit would bring about increment of the cost.